Regenerative braking systems are employed for energy savings. Many mechanical systems operate with variable kinetic energy with repeated start/stop cycles. A common example of such a system is a motor vehicle in stop and go city traffic. Other vehicular examples include garbage trucks and warehouse lift trucks. These systems are not limited to vehicles but include industrial processes as well. A simple example is the start/stop operation of high speed thread bobbin winders. The common feature of such applications is that the energy expended in accelerating to a "cruise speed" is comparable to that used to maintain the "cruise speed" for the cruise duration. Typically the deceleration process employs a mechanical brake. Thus much of the energy consumed in the system is dissipated in brake heating. Regenerative braking systems are proposed to capture and store some of the vehicular kinetic energy typically dissipated during braking. Regenerative braking systems then employ some of this stored energy in the next acceleration cycle.
The major problem with prior regenerative braking systems is the efficiency of energy storage and conversion, and control of the energy transfer process. Prior proposed regenerative braking systems have included: electrical systems which include a motor/generator and a chemical battery; flywheel systems; hydrostatic systems employing compressed air; and mechanical strain systems employing steel or elastomeric springs. These systems suffer from disadvantages which preclude their widespread use.
The prior electrical systems suffer from a limited capacity to absorb and deliver energy. In such systems much energy is lost during braking because chemical batteries of reasonable size cannot absorb energy at the rate generally required during deceleration, particularly for vehicles. Further, the same battery typically cannot deliver energy efficiently at the rate required by the system during acceleration.
Flywheel systems present difficult control problems. This is because the rate of change of speed of the flywheel must be opposite that of the mass itself. Thus, for example, when the vehicle is decelerating the flywheel must be rotated faster to store more energy. Likewise, when the vehicle is to accelerate the flywheel must be slowed to deliver energy. This leads to a requirement for a complex wide range variable ratio transmission. In addition, gyroscopic effects prevent use of this technique is some vehicles.
Hydrostatic systems provide energy storage in compressed air. The round trip efficiency of the required pump for energy storage and pneumatic motor for energy delivery are of the same order as flywheel systems. Such systems can be relatively compact. They are disadvantageous in that they require moderately complex control systems, though of a lesser degree than flywheel systems. Much energy is lost in heat in such systems.
Strain systems employing either steel or elastomeric springs have an advantage in that they form naturally oscillating systems. A mass on a spring has a natural tendency to exchange kinetic energy for potential energy and vice versa. Thus no complex energy conversion machinery is required because the system naturally exchanges energy in the manner desired. One problem with such a naturally oscillating system is that it has its own inherent energy exchange rate which can only be changed by changing either the mass or the effective spring constant of the system. Thus energy exchanges at widely differing rates is difficult with such systems.
There is a need in the art of regenerative braking systems for a system with a high round trip efficiency, which requires a minimum of controls and which permits variable energy transfer rates.